For improvement of control accuracy of optical devices, improvement of accuracy of a film thickness of an optical thin film is in demand. For the highly accurate film thickness control of the optical thin film, measurement is indispensable, and various types of film thickness measurement methods and film thickness meters used for the film thickness control have been proposed. It is preferable that an optical film thickness meter excellent in responsiveness and the like is used for the film thickness measurement. The film thickness here means the film thickness of an optical thin film and has a value depending on the physical film thickness and refraction index.
The optical film thickness meter can be roughly divided into a reflection type and a transmission type. The reflection type is a technology for measuring the film thickness by using a phenomenon of interference caused by generation of a phase difference due to a difference between a path of a light beam reflected on the surface of the optical thin film and a path of a light beam reflected on an interface between a substrate and the optical film, and since reflectance as the entire light beam cyclically changes with respect to the film thickness, this technology is often used if the number of formed films is small or if a relative measurement can be used and has nonconformity in which use applications are relatively limited.
On the other hand, the transmission type is, as illustrated in FIG. 9, a technology in which light emitted from a projector 11 is reflected on a mirror 105 disposed in a mirror box 107 so that the light beam transmitted through the optical thin film is measured, and both the film thickness and spectroscopic characteristics can be acquired from the transmissivity of the light amount. Since this technology is hardly susceptible to a change in the light amount caused by a change in an angle of an actual substrate S, it has an advantage that measurement can be performed with high accuracy.
However, since a substrate for a monitor is used and the substrate for a monitor is arranged at a position different from a position of the actual substrate, a film-thickness difference is present between the substrate for a monitor and the actual substrate, and an experience and knowledge of a staff in charge of film formation are needed for correcting the film-thickness difference, which results in an instability factor in a film forming process and also nonconformity of occurrence of a film thickness control error.
Also, the prior-art optical film thickness meter is mounted on a thin film forming apparatus 3 as illustrated in FIG. 9, and it is difficult to improve measurement accuracy for a film with low refraction index. For example, SiO2 often used as a deposition material has a small refraction index difference from monitor glass, and particularly in a transmission photometric system, a change amount in light amount in the in-situ measurement is small and control is difficult, which are disadvantageous. That is, if the change amount in light amount is small, control is forced to be executed on the basis of a limited change amount, and improvement of accuracy is made difficult.
In order to solve the above problems, a technology in which a light beam (outgoing light) having passed through a measurement substrate is reflected by a corner cube prism and the light beam (reflection light) having passed through the measurement substrate again is measured so as to measure the film thickness and the like is proposed (See Patent Document 1, for example).
Patent Document 1: Japanese Patent Laid-Open No. 2006-45673